Chromatin structure is related to gene activity in that transcriptionally active chromatin is more susceptible to DNAase I than inactive one. In addition, there are sites at 5' flanking region of a gene which are extremely sensitive to DNAase I when the gene is active. Absence of, or base changes at these sites is correlated with the absence or reduction of gene expression. We propose to study such nuclease sensitivity in the rosy locus of Drosophila melanogaster. The rosy locus contains the structural gene of xanthine dehydrogenase (XDH) and its DNA has been cloned. Its expression is developmentally regulated and tissue specific. There are several control mutants including heterochromatic position effect mutants available in this locus. We have identified base changes in the control region of two underproducer mutants by DNA sequencing. We wish to analyze the DNAase I hypersensitivity of this locus in wild types and control mutants. Any variations we observe will be correlated with the known DNA sequence and the XDH expression. These informations should also be useful for directed mutagenesis work in this locus. With the heterochromatic position effect mutants, we will examine, in addition, a general DNAase I sensitivity of active chromatin. We hypothesize that heterochromatic position effects are due to heterochromatin condensation in such a way that an adjacent gene in eucromatin shows a reduced expression. Such condensed chromatin could be more DNAase resistant. Collaborative efforts of our project with W. Bender's and A. Chovnick's laboratories are planned. It is hoped that our joint efforts will contribute in understanding the control of the XDH expression and regulation.